1. Field of the Invention
The present invention relates to a direct current power supply device, and further, to a voltage regulator that converts a direct current voltage. For example, the present invention relates to a technology effective for use in a semiconductor integrated circuit (regulator-ready IC) that composes a series regulator provided with a soft start function and an overcurrent protection function.
2. Background Art
In a series regulator, for example, when an overcurrent flows out of an output terminal by the matter that a load is short-circuited, and so on, there is an apprehension that a current controlling transistor may generate heat to raise a chip temperature of an IC, causing such defects that an internal circuit malfunctions, that an element is broken, and so on.
Heretofore, in the series regulator, in order to protect such a chip from the overcurrent as described above, a current limit circuit has been provided, which has an overcurrent protection function to make control so that, when an output current Iout exceeds a predetermined value, output voltage-output current characteristics represented by a shape of a so-called “reverse C” can be established by reducing the output current Iout while lowering the output voltage Vout, for example, as shown in FIG. 9A (Japanese Patent Laid-Open Publication No. 2008-052516).
Moreover, there have also been proposed inventions regarding a voltage regulator, which is composed so as to provide a soft start circuit separately from the current limit circuit and in combination therewith in order to restrict a so-called rush current as an output current flown into a capacitor at once when a power supply is turned on (Japanese Patent Laid-Open Publication No. 2002-049430, Japanese Patent Laid-Open Publication No. 2010-170363).
FIG. 7 shows a schematic configuration of a conventional voltage regulator in which the soft start circuit and the current limit circuit are provided. In FIG. 7, reference numeral 21 denotes the soft start circuit, and reference numeral 22 denotes the current limit circuit. The current limit circuit 22 has the same circuit configuration as an overcurrent protection circuit disclosed in Japanese Patent Laid-Open Publication No. 2008-052516. A size of transistors which compose the circuit is adjusted, whereby a current restriction function, which is in accordance with a drooping type voltage-current characteristics as shown in FIG. 8A or the reverse-C type voltage-current characteristics as shown in FIG. 9A, can be imparted to the current limit circuit 22.
The soft start circuit 21 shown in FIG. 7 includes: a time constant circuit composed of a constant current source CI and a capacitor C0; a comparator CMP that compares a voltage Vst of the time constant circuit with a voltage VFB obtained by dividing an output voltage Vout by bleeder resistors R1 and R2; and a switching switch SW capable of switching a voltage of the time constant circuit and a reference voltage Vref and supplying the voltage and the reference voltage Vref to an error amplifier AMP.
Then, at the time when the power supply rises, the voltage Vst of the time constant circuit is supplied to the error amplifier AMP, and the output voltage Vout is raised slowly. When the output voltage Vout reaches a certain potential, then the switch SW is switched to supply the reference voltage Vref to the error amplifier AMP, and control to hold the output voltage Vout at a constant voltage is performed.
As shown in FIG. 7, in the conventional voltage regulator, the soft start circuit and the current limit circuit are composed as separate circuits. Accordingly, a circuit scale of the voltage regulator is large, and in the case of forming the voltage regulator into a semiconductor integrated circuit, there has been a problem that an increase of a chip size, and eventually, an increase of cost are brought about. Moreover, in the conventional current limit circuit, in general, the characteristics thereof are of the drooping type shown in FIG. 8A or of the reverse-C type shown in FIG. 9A, and in power consumption-output current characteristics thereof, as shown in FIG. 8B or FIG. 9B, such power consumption takes a relatively high value in a course after the overcurrent is detected. Accordingly, there are problems that a power loss is large, that the chip temperature temporarily rises to an allowable level or more, and the like.